Sensored brushless motor technology is well-known and is useful for minimal flaw control at low speeds and reliable rotation. A sensored system has one or more sensors that continuously communicate with a motor controller, indicated to it what position the rotor is in, how fast it is turning, and whether it is going forward or reverse. Sensors in a sensored system increase cost and provide additional pieces that can break or wear down, adding durability and reliability issues. Sensorless systems can read pulses of current in the power connections to determine rotation and speed. Sensorless systems tend to be capable of controlling motors at higher speeds (e.g., revolutions per minute (“RPM”)), but may suffer “jitters” under a load at very low starting speeds, resulting in a performance inferior to sensored brushless motors.
Jitter is a phenomenon that occurs with sensorless brushless motor systems at initial starting speed and generally no longer exists after the motor has gained sufficient speed. Jitter comes about because at low or zero speed, the sensorless algorithm does not have enough information to decide which windings to energize and in what sequence. One common solution for starting a sensorless system is to energize one winding pair to lock the rotor at a known position. The motor windings are then commutated at a pre-defined rate and PWM duty cycle until the rotor reaches a speed sufficiently high for the sensorless control to engage. However, even this solution will cause jitter during startup, particularly if there are time varying loads. Jitter can be decreased or made imperceptible for loads with minimal initial torque or predictable initial torque. However, some motor application/use situations (such as starting an electric motor bike moving uphill) demand significant torque for initiation, and the initial torque is highly unpredictable. Use of sensorless brushless motor systems is sometimes discouraged for low-speed high-torque maneuvers, like rock-crawling or intricate and detailed track racing of an electric motor vehicle/bike, because in such difficult situations, significant jittering may occur and can lead to premature motor burnout.
FIG. 1 is a block diagram of a motor control system 10 in a three-phase power stage, as is known in the prior art. Many three-phase motor control systems 10 include a controller having a control signal generator 12, a gate driver 14, and a power stage 16. In case of sensorless control, feedback circuits are also included, specifically a detection network 18 and a current sensing circuit 20, which utilizes sense resistor RSENSE. In general, a goal of sensorless control is to detect a motor response to an applied pulse width modulated (PWM) source voltage to identify rotor position and movement.
Similarly, a current sense circuit 20 may be used to detect the magnitude and direction of motor current across driven windings. Low side shunt monitoring is used regularly. An often used configuration for low side monitoring is shown in FIG. 1. One skilled in the art can easily adopt alternative current sensing techniques such as monitoring phase current in each inverter branch including high-side monitoring and this alternative technique is known to those having ordinary skill in the art.
The control signal generator 12 is often powered from a low voltage source. As a result, a function of the gate driver 14 includes shifting the low voltage control signals to levels that match input requirements of the power stage 16. The power stage 16 is includes semiconductor switching devices. MOSFETs are shown in FIG. 1, but other devices such as IGBTs may be used. The control signal can be made to generate trapezoidal (a.k.a. block or 6 step commutation) or sinusoidal drive from the power source Vpwr. Pulse width modulation is typically used with trapezoidal drive in brushless DC (BLDC) motor control. Systems requiring lower audible noise or lower torque ripple benefit from sinusoidal drive.
Those skilled in the art with respect to PWM drive techniques understand a variety of modes to generate trapezoidal, sinusoidal, or other control. The motor response to a PWM drive can be detected via voltage on the motor phases and/or phase current(s).
As shown in FIG. 1 for a brushless DC motor control, the power stage 16 is driven such that current flows into a first motor phase (for example, phase U) and exits a second motor phase (for example, phase V). The rotor (not shown) position within the motor 30 dictates which phase pair to drive to attempt to achieve full torque and smooth (jitter-free) rotation of the rotor. The feedback controls are used to deduce rotor position.
FIG. 2 is an illustration of a wye-connected motor 30, as is known in the prior art. The wye-connected motor 30 in this illustration has a single-pole pair permanent magnet rotor 32 positioned such that its south pole 34 is proximate to the winding of the U-phase 36. Under these conditions, it is obvious to one skilled in the art that the W-phase 38 and the V-phase 40 are the appropriate phase pair to drive in order to initiate rotation of the rotor 32. The polarity of the permanent magnet rotor 32 determines the direction of current flow through the phase. Hence, the power stage 16 connects the W-phase 38 to Vpwr and the V-phase 40 to ground 24 resulting in current flow into the W-phase 38 and exiting the V-phase 40, as represented with the current arrows. A net effect of current flowing through coils W-phase 38 and V-phase 40 as shown in FIG. 2 is the formation of an electromagnet having a north pole at the W-phase 38 and a south pole at V-phase 40. This electromagnet produces a repulsive force between permanent magnet N-pole 42 and the electromagnet N-pole formed at the W-phase 38 and an attractive force between permanent magnet N-pole 42 and the electromagnet S-pole formed at the V-phase 38.
As N- and S-poles are attracted to each other, if the electromagnet persisted long enough in this current flow configuration, the resulting torque will move the permanent magnet N-pole 42 to a position shortly after the V-phase 40 and the permanent magnet S-pole 34 to a position shortly before the W-phase and rotation of the permanent magnet rotor 32 would stop. To perpetuate rotation of the permanent magnet rotor 32, the power stage 16 must commutate to a new phase pair. The optimum commutation point is a function of the rotor position relative to the coil of the undriven phase (the phase not driven by Vpwr). In FIG. 2, the U-phase 36 is the undriven phase. Ideally, the rotor angle would span −30° to +30° with respect to alignment with the coil of the undriven phase. As this 60° span is one sixth of one electrical revolution, it is commonly referred to as one sextant.
FIG. 3 is a 6-step commutation process further defined by Table 1, as is known in the prior art. Given the conditions illustrated in FIG. 2, a high level description of the sequence of steps commonly referred to as 6-step commutation process is outlined in Table 1 and further illustrated in FIG. 3.
TABLE 1Six-step commutation sequence for awye-connected motor shown in FIG. 2Se-DrivenquencePhaseN-pole positionS-pole positionRotorStepPairrelative to phasesrelative to phasesAngle0WVW + 30° to V − 30°U − 30° to U + 30°1.25-1.751WUV − 30° to V + 30°U + 30° to W − 30°1.75-2.252VUV + 30° to U − 30°W − 30° to W + 30°2.25-2.753VWU − 30° to U + 30°W + 30° to V − 30°2.75-0.254UWU + 30° to W − 30°V − 30° to V + 30°0.25-0.755UVW − 30° to W + 30°V + 30° to U − 30°0.75-1.25
The 6-step commutation sequence results in one electrical revolution. Given this simplified example, it is understood that a properly driven permanent magnet rotor will be driven one mechanical revolution when this six-step process is complete. An increase in number of pole pair results in an equivalent increase in the number of electrical revolutions per mechanical revolution. Comparing Table 1 and FIG. 2, it is understood that FIG. 2 illustrates Sequence Step 0 with the permanent magnet N-pole 42 pushed from the W-phase 38 and pulled by the attraction to the V-phase 40. When the permanent magnet S-pole 34 reaches the U+30° position, the power stage 16 commutates to Sequence Step 1 driving current from the W-phase 38 to the U-phase 36 causing the U-phase to become the electromagnetic S-pole. Thus, the U-phase 36 repels or pushes the permanent magnet S-pole 34 and the W-phase 38 attracts the S-pole, continuing the clockwise motion of the permanent magnet rotor 32.
Most current solutions to sensorless control of a brushless permanent magnet motor utilize a symmetric pulse width modulation signal. FIG. 4A is an illustration of one example of one symmetric pulse width modulation signal, as is known in the prior art. One cycle of a symmetric pulse width modulation signal may include a positive voltage V+ for a span of time TA and then a negative voltage V− for the span of time TB, where the absolute values of V+ and V− are equivalent and a full PWM period is TA+TB. The span of time spent at V+ is referenced as the energizing portion of the signal and the span of time spent at V− is referenced as the de-energized portion of the signal. FIG. 4B is an illustration of one example of one asymmetric pulse width modulation signal, as is known in the prior art. This signal includes a span of time T1 at V+ and a second span of time T2 at approximately 0V. The sum of T1 and T2 represents the PWM period. It would be useful to have a control system for a brushless permanent magnet motor that could control a motor operating from an asymmetric pulse width modulated signal.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.